In the manufacture of linalool from pinanol (III) by pyrolysis of the latter, plinols constitute the major loss. Plinols which are monocyclic compounds (1,2-dimethyl-3-isopropenyl cyclopentanols) (I) are obtained from an intramolecular -ene reaction of linalool (II) (Ikeda et al, J. Chem. Soc. Jap. 57, page 425 (1936); CA 30, 5937 (1936)). According to Ohloff et al, Helv. Chim. Acta 50, 759 (1967), the above reaction yields four plinol isomers in the approximate percentages indicated in the following equation: ##STR1##
The formation of plinols from linalool in the pyrolysis reaction cannot be avoided because the activation energy for this secondary reaction is lower than that of linalool generation. The amount of this "waste" is substantial. At the outset of this work, no practical utility for the plinols had been found other than as a fuel supplement.
Pipertone (IV) occurs in a variety of eucalyptus oils. Hydrogenation of piperitone (IV) furnishes menthone which is readily converted to menthol. Thus a process productive of piperitone represents also a portion of a menthol process.
It has now been found that piperitone (IV), can be prepared from plinols by a cyclomutation sequence whereby what was normally a waste product, is convertable to a material useful, for example, as a precursor to menthone, or mixed isomeric menthols by hydrogenation.
As will be seen from the following disclosure, the unit organic operations employed herein include hydrogenation, dehydration, oxidative cleaving of an unsaturated ring, cyclization and dehydration in that order. Hydrogenation is a well known unit organic operation, and the catalysts and conditions of temperature and pressure useful in effecting the addition of hydrogen to unsaturates are well known. (See Streitwieser et al "Introduction to Organic Chemistry" 1976, page 276). Dehydration to move the formerly external unsaturation into the ring is also known per se. (See ibid p. 272-275). Ozonolysis is also a known unit organic operation (See ibid pages 290-292). Mutative cyclization and dehydration of an aliphatic dione with an alcoholic solution of a strong base is also known (See "Organic Reactions", Volume 16, Pages 49-58).
Reference may also be had to Kutney et al, Tetrahedron Letters, 2911 (1965) Anderson et al, JACS, 73, p 232 (1951); Johnson et al, JACS 93, p. 4332 (1971); and Marshall et al, JACS 84, p. 1485 (1962) showing certain of the operations in sequence as applied to polycyclic materials, e.g., alkaloids, decahydro-2-naphthtol-1, production of progesterone from a trienynol, and synthesis to a cyclic olefin which is oxidized and then cyclized via an aldol condensation. Regarding the general scheme of the invention, related transformations may be found in the above Kutney et al reference. However, there is nothing in the literature that suggests the application of the present multi-step process to the synthesis of piperitone from plinols. Piperitone produced in accordance herewith may be hydrogenated according to the procedure described by Blagden et al in U.S. Pat. No. 2,237,980.